No, don't vent back into the grow room, I don't think that could even be considered venting. If you don't move air out of the grow you won't pull new air into the grow. If you don't pull new air in, you need to maintain CO2 levels artificially and you have to handle humidity build up in the room. If you vent to an outside room you will circulate the air and you can handle humidity issues outside of your grow space. Put the dehumidifier "up wind" from your grow intake.
I vent my grow to the room above it and have the dehumidifier where-ever I want it in the house. I only grow in winter as I use the grow for heat and can't maintain reasonable temps in summer or warmer parts of spring/fall. Plus there is a whole world of free light outside to grow as much as I want come summer. (Start hiking, it can be rewarding, evil smile)